1. Field of the Invention
The present invention relates to an optical switch element and a liquid crystal light directional coupler which can be used in an optical transmission apparatus or an optical information processing apparatus.
2. Description of the Related Art
Now, a flat panel display device which performs a display based on a different principle from that of a conventional cathode ray tube (CRT) is widely used as a display in a document editing apparatus such as a word processor, or in an electronic apparatus such as a personal computer. Developments are pursued in the future application of such a flat panel display device as a display used in a high definition television receiver or in a high-performance engineering work station (EWS).
As such a flat panel display device, an electroluminescence panel (ELP), an plasma display panel (PDP), a liquid crystal display (LCD) or the like is known. Among these known display devices, the LCD is the most promising in terms of high accommodation to full color display, the application in conjunction with a large scale integrated circuit (LSI), and the like.
There two types of LCDs depending on the construction and the method for driving the display medium. One is an LCD of simple matrix driving type. The other is an LCD of active matrix driving type. In the LCD of simple matrix driving type, on surfaces of a pair of glass substrates which face each other, stripe-like electrodes are formed, respectively. The glass substrates are combined in such a manner that the stripe-like electrodes on the glass substrates are perpendicular to each other. Into a space between the glass substrates, nematic liquid crystal is injected. A display is realized by using the abrupt changing ability in the optical characteristics between the display state and the non-display state of the nematic liquid crystal.
The LCD of active matrix driving type has a construction in which pixels are used with nonlinear elements. The LCD of active matrix driving type performs the display by using the switching characteristics of the respective nonlinear elements. Therefore, as compared with the LCD of simple matrix driving type, the LCD of active matrix driving type has a lower dependence on the abruptness of changing in the optical characteristics of the liquid crystal, thereby realizing a display device with a high contrast and high-speed response.
The nonlinear elements are classified into two types. One is a two-terminal element, and the other is a three-terminal element. As the two-terminal type nonlinear element, an MIM (metal-insulator-metal) element, a diode and the like are known. As the three-terminal type nonlinear element, an a-Si TFT (amorphous silicon thin film transistor) element, a p-Si TFT (polysilicon thin film transistor) element, and the like are known.
In the LCD of active matrix driving type, parasitic capacitances are generated in the nonlinear elements and scanning lines. This causes problems such that the decrease in contrast, the occurrence of persistence, the reduction in panel lifetime, and the like. In the recent future, the display panel will be increased in size. The large-sized panel necessitates longer wirings. As a result, the wiring resistance is increased, and the delay in signal is increased by the combination of the parasitic capacitance and the wiring resistance. This causes a problem in that the uniformity and the high contrast of the display are further degraded.
In order to eliminate the above problems, the waveforms of the driving pulses for the display in the respective pixels are analyzed. However, it is difficult to logically analyze the waveforms in detail due to the nonlinearity of the switching elements.
On the other hand, as is shown in FIG. 7, light from a light source 41 to an optical wave guide 42 is branched by light branching elements 43, so that the branched light beams are transmitted through a plurality of row optical wave guides 44, respectively. If the above optical scanning manner is used, the above-mentioned parasitic capacitances between the nonlinear elements associated with the pixels and the electrical wirings can be remarkably reduced. Therefore, by using the above optical scanning manner, the above problem of signal delay is eliminated, whereby the display device can be readily made larger. The light branching element 43 is an optical switch element which passes or shields the light from the optical wave guide 42 to the row optical wave guide 44.
There are two types of optical switch elements. One is of mechanical type in which an optical path is switched by mechanically driving a fiber, a prism or the like. The other one is of electronic type in which an electro-optic effect and an acousto-optic effect of LiNbO.sub.3, As.sub.2 S.sub.3, etc. are utilized. An optical switch element of the electronic type has no mechanically movable section, so that it offers high reliability. Especially, in recent years, an optical switch element of the electronic type which uses liquid crystal is attractive. For example, there is an optical switch element of the electronic type, i.e., a so-called light directional coupler in which liquid crystal and an optical wave guide are used in combination, which is described in Japanese Laid-Open Patent Publication No. 57-142622 (Applicant: Nippon Telegraph and Telephone Public Corporation).
FIGS. 2A and 2B are cross-sectional views for illustrating a structure and operation of a conventional optical switch element using liquid crystal. Referring to these figures, a first optical wave guide 2 is formed on a glass substrate 1, and a common electrode 6 is formed thereon. A second optical wave guide 5 is formed on a counter glass substrate 4, and a segment electrode 7 is formed thereon. A liquid crystal layer 3 is formed between the common electrode 6 and the segment electrode 7. The common electrode 6 and the segment electrode 7 are made of transparent conductive films. These electrodes are sufficiently thin, so as not to affect a relationship between each of the optical wave guides 2 and 5 and the liquid crystal layer 3, i.e., total reflection or scattering.
FIG. 2B shows the light transmission direction from the first optical wave guide 2 to the second optical wave guide 5, when voltage is applied to the segment electrode 7 and the common electrode 6 of the optical switch element. In this case, refractive indexes of the respective component materials of the optical switch element satisfy first conditions of n.sub.A0 &lt;n.sub.1 &lt;n.sub.e1 &lt;n.sub.2, and n.sub.2 &gt;n.sub.B0, or second conditions of n.sub.e1 &lt;n.sub.1, n.sub.e1 &lt;n.sub.2, n.sub.1 &gt;n.sub.A0, n.sub.2 &gt;n.sub.A0, n.sub.1 &gt;n.sub.B0 and n.sub.2 &gt;n.sub.B0 (where n.sub.e1, n.sub.A0, n.sub.1, n.sub.B0 and n.sub.2 are refractive indexes of the liquid crystal layer 3, the glass substrate 1, the first optical wave guide 2, the counter glass substrate 4 and the second optical wave guide 5, respectively). At a first scattering, 99.9% of an incident light beam a.sub.0 is transmitted out of the first optical wave guide 2 (such a light beam is referred to as a'.sub.1). The remaining 0.1% of a.sub.0 is still in the first optical wave guide 2 (such a light beam is referred to as b'.sub.1). At a second scattering, 99.9% of b'.sub.1 is scattered and 0.1% of b'.sub.1 is still in the first optical wave guide 2. After the second scattering, 0.0000001% of a.sub.0 (b'.sub.out) remains in the first optical wave guide 2, and 99.9999999% of a.sub.0 is scattered. The scattered light is attenuated to be about 70% due to a propagation loss in the liquid crystal layer 3 or the like. The attenuated light (a'.sub.out) is guided to the second optical wave guide 5.
FIG. 2A shows a case where no voltage is applied to the optical switch element. In this case, refractive indexes of the respective component materials satisfy conditions of n.sub.A0 &lt;n.sub.1, n.sub.1 &gt;n.sub.e1, n.sub.B0 &lt;n.sub.2 and n.sub.2 &gt;n.sub.e1. The refractive index n.sub.e1 is smaller than that in the case where voltage is applied. Therefore, the incident light beam a.sub.0 is repeatedly and totally reflected in the first optical wave guide 2. At each total reflection, 0.1% of a light beam is scattered. Although the light is gradually attenuated by 0.1% in the order of a.sub.1, a.sub.2 and a.sub.3, approximately 100% of the incident light beam a.sub.0 (a.sub.out) is guided in the first optical wave guide 2. The scattered light beams (about 0.1%) b.sub.1, b.sub.2 and b.sub.3 scattered at an interface between the first optical wave guide 2 and the liquid crystal layer 3 are attenuated to be 0.07% due to the propagation loss in the liquid crystal layer 3. The scattered light beams b.sub.1, b.sub.2 and b.sub.3 are guided to the second optical wave guide 5 and are sequentially summed up, so as to be totally 0.2%, which constitutes leakage light.
Because of the leakage light, the conventional optical switch element has the S/N ratio of 26 dB. Therefore, the conventional optical switch element has a problem of crosstalk in which the optical signal transmitted through the first optical wave guide 2 is leaked at the optical switch element to the second optical wave guide 5. Moreover, since the conventional optical switch element uses nematic liquid crystal, there is another problem in that the switching rate is several milliseconds, i.e., the response speed is relatively low.
Another prior art for optical scanning uses a light directional coupler as the above-mentioned light branching element. The light directional coupler is mainly used for branching an optical signal in the optical communication field. For the optical wave guide, an optical fiber, glass optical wave guide, a semiconductor optical wave guide or the like is used. As a material for the light directional coupler type switch element, Ti:LiNbO.sub.3 is known. Moreover, a (4.times.4) or (8.times.8) matrix switch of cross connection type in which light directional couplers are integrated is now being developed.
Still another prior art uses a liquid crystal light directional coupler as the light branching element. By the conventional liquid crystal light directional coupler, a coupling length is about several millimeters. Therefore, when the optical scanning is performed in the display device, and the size of each of the liquid crystal light directional coupler is assumed to be equal to the above length, the row optical wave guides extending along a row direction across the liquid crystal light directional couplers from the main optical wave guide extending along a column direction cannot be formed in high density, which constitutes a problem.